WIP Backends is a design artifact of WebKit Remote Debugging Protocol client implementation. It allows to have several different protocol implementations behind the single SDK interface. Technically WIP backend is a sub-library that should be used together with the main SDK library. There are several of them.
This is how Java SDK tolerates the fact that different versions of Google Chrome/Chromium simultaneously available may have incompatible protocols.
WIP Backend is currently a user-visible detail. This means that it's a user responsibility to correctly select a proper WIP Backend before attaching from debugger. Each backend is presented in Debugger UI in Debug Configurations dialog with a small declaration of compatible browser versions.
The exact set of backends change from release to release. That's why they are normally listed in release notes.
WIP Backends are deployed as a separate Eclipse feature "ChromeDevTools SDK WIP Backends".
Wiki: DebuggerTutorial
Wiki: EclipseDebugger
Wiki: Release_0_3_6
Wiki: WIP
Wiki: WebKitProtocol
Wiki: WipBackendDevelopment